Compensation Effect in Transesterification Kinetics of Croton Megalocarpus Oil using Heterogeneous Alkaline Earth Catalysts

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056
Volume: 04 Issue: 03 | Mar -2017 www.irjet.net p-ISSN: 2395-0072
© 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 1568
Compensation Effect in Transesterification Kinetics of Croton
Megalocarpus Oil using Heterogeneous Alkaline Earth Catalysts
Anil Kumar, Saul S. Namango
Department of Chemical & Process Engineering
Moi University, Eldoret, Kenya
----------------------------------------------------------------------------------***---------------------------------------------------------------------------
Abstract - Reaction kinetics of transesterification of
croton megalocarpus oil with methanol to produce fatty
acid methyl ester was studied at 70oC, using homogeneous
and heterogeneous catalysts through conventional heating
and microwave irradiation. NaOH was used as a
homogenous catalyst, and alkaline earth metal oxides, BaO,
SrO, CaO, MgO and BeO, were the heterogeneous catalysts.
Reaction rate constant k; and Arrhenius equation
parameters, pre-exponential factor A and activation energy
E were estimated from kinetic data, using the overall
reaction. By considering the magnitudes of k, NaOH was the
most active catalyst, followed by BaO, SrO, CaO, MgO and
BeO. However, no specific order was observed in the
estimated values of A and E. Activation energy was not
found to be a direct measure of catalyst activity for
heterogeneous catalysts. This was explained by the complex
nature of the three-phase reaction system where twenty one
reactions were likely to be taking place simultaneously, and
the observed kinetic parameters were the overall apparent
values. Cremer Constable scatter plot showed the data
points roughly following a straight line. It was concluded
that the discrepancies in A and E values were due to
oversimplification of kinetic model, rather than being a
physical phenomena.
Key Words: Transesterification, Croton megalocarpus,
Alkaline earth oxides, Kinetics, Conventional heating,
Microwave irradiation, Compensation effect, Cremer
Constable plot.
1. INTRODUCTION
Biodiesel is a biofuel obtained from renewable sources and
is recognized as an alternative to petro-diesel. In 2014
biofuels provided 4% of the fuel for transport sector,
which is to rise to 4.3% in 2020. Reduction in petroleum
crude prices in 2014 affected the production and use of
biofuels. Production in 2014 was 126 billion litres,
expected to rise to 144.5 billion litres in medium terms to
year 2020 [1]. In spite of the recent discoveries of new
crude sources, especially in Africa; petroleum reserves are
finite, and would deplete or exhaust. It is estimated that
the world is consuming 2.7% of petroleum reserve
annually, and the stocks would last about 50 years [2].
Another advantage of biofuels is their superior
environmental footprint. Biofuels consist of mainly carbon,
hydrogen and oxygen. Upon combustion, these fuels are
carbon neutral, do not produce nitrogen and sulphur
oxides, unlike petroleum derived fuels. Biodiesel fuel
produces far lower emissions of unburned-hydrocarbons,
carbon dioxide, carbon monoxide, sulphates, polycyclic
aromatic hydrocarbons, nitrated polycyclic aromatic
hydrocarbons, ozone forming hydrocarbons and
particulate matters [3]. Greenhouse gas emissions from
biodiesel are between 22 and 59% of the emissions from
petro-diesel. [4]. Biodiesel is produced by reacting
vegetable oils or animal fats with an alcohol in presence of
a catalyst. The reaction is a transesterification reaction
producing alkyl ester (biodiesel) and glycerol.
1.1 Transesterification reaction
In a transesterification reaction of a triglyceride (oils and
fats) alcohol is displaced from the triglyceride ester by
another alkyl alcohol to produce an alkyl ester (biodiesel)
and glycerol. The overall reaction can be represented as
follows:
Triglyceride + 3 ROH ↔Glycerol + 3 alkyl ester (i)
When methanol is used as alkyl alcohol, the corresponding
alkyl ester is fatty acid methyl ester (FAME). The above
reaction is a reversible reaction, and is usually carried out
in presence of a catalyst. Reaction (i) is the overall
reaction, supposed to take place in three steps. In the first
step, triglyceride reacts with alcohol to produce
diglyceride and ester. Further reaction with alcohol
produces nonoglyceride and ester (second step), followed
by reaction of monoglyceride with alcohol to produce

glycerol and ester (third step). The three reaction steps are
as below:
Triglyceride + R'
OH diglyceride + R'COOR1
catalyst
Diglyceride + R'OH monoglyceride + R'COOR2
catalyst
glycerol + R'COOR3
catalyst
Monoglyceride + R'OH
...(ii)
...(iii)
...(iv)
In the above reactions, R' is the alkyl group for the alcohol
whereas R1 , R2 and R3 are carbon chain of fatty acid [5].
Catalysts used for the transesterification reaction can be
homogenous or heterogeneous. Homogeneous catalysts
are acids, bases and enzymes. Homogeneous basic
catalysts such as sodium hydroxide, alkali metal alkoxides
and potassium hydroxide, are generally preferred due to
their higher activity. Homogenous catalysts cannot be
recovered during the process, and therefore cannot be
recycled. Neutralizing and washing of theses catalysts are
a major process steps, which adversely affects the
economic viability and environmental impact.
Heterogeneous catalysts, on the other hand, can be
recovered and recycled, and are more environmental
friendly.
1.2 Heterogeneous catalysts
Heterogeneous catalysts are classified as single component
metal oxides, zeolites, supported alkali metals, clay
minerals and non-oxides [6]. Theses catalysts are highly
selective and their activity mostly depends on their surface
properties [7]. Similar to homogenous basic catalysts,
heterogeneous basic catalysts too give a higher reaction
rate. Single component basic metal oxides have been
extensively studied for transesterification. Alkaline earth
metal oxides, BaO, SrO, CaO, MgO, BeO have been used in
pure form, or supported on a porous base. Surfaces of
these basic oxides are covered with water and carbon
dioxide, and they also form carbonates and hydroxides
when exposed to atmosphere. A pre-treatment at high
temperature is usually required to remove adsorbed
species, and to break hydroxides and carbonates back into
oxides [8], [5]. Transesterification reaction kinetics were
studied for croton megalocarpus oil and methanol reaction
employing BaO, SrO, CaO, MgO and BeO catalysts.
1.3 Reaction kinetics with alkaline earth metal
oxide catalysts
Reaction system consists of three immiscible phases- oily,
aqueous and solid; composed of triglyceride, FAME,
glycerol, methanol, and catalyst. This poses severe mass
transfer and surface reaction challenges. In a solid-liquid-
liquid system, the resistance to reaction could be due to
seven steps [9]: (1) Diffusion of reactant molecules
through bulk liquid phase to catalyst surface, (2) Diffusion
of reactant molecules through liquid-solid surface or
catalyst pore, (3) Adsorption of reactant molecules on
catalyst active sites, (4) Chemical reaction at catalyst
surface, (5) Desorption of product molecules from catalyst
active sites (6) Diffusion of product molecules through
liquid-solid surface or catalyst pore, (7) Diffusion of
reactant molecules through catalyst surface to bulk liquid
phase. Slowest of these seven steps, controls the reaction
rate. These seven steps occur in the three
transesterification reactions, (ii), (iii) and (iv) given above,
resulting into overall twenty seven reaction steps.
Langmuir-Hinshelwood mechanism for a solid- fluid
reaction, for two reactants and two products, consists of
five reaction steps, chemisorption of the two reactants,
surface reaction, desorption of the two products [6].
Another mechanism for solid-fluid reactions is Eley-Rideal
mechanism which has similar reaction steps. For
transesterification reaction, Langmuir-Hinshelwood
mechanism is preferred [10]. This emphasises the complex
nature of reaction steps, which makes it practically
impossible to study a single intermediate reaction. The
observed reaction parameters are, therefore, the net result
of these numerous transport/ reaction steps.
1.3.1 Reaction rates, activation energy and pre-
exponential factor
Chemical reaction rate constant, k , is a temperature
dependent term. The temperature dependency is well
represented by Arrhenius’ law [11]:
k = ko e–E/RT …(v)
where ko is called the frequency factor (or pre-exponential
factor, A) and E is called the activation energy of the
reaction. T is the temperature and R is universal gas
constant.
An alternate expression of Arrhenius’ law is:
k = ko′ Tm e–E/RT 0 ≤ m ≤ 1 …(vi)
The exponential term in the above equation (vi) is so much
more temperature-sensitive than the Tm term, the
variation of k caused by the later is effectively masked, and
the expression reduces to equation (v). Equation (v), in
logarithmic form is:
ln k = ln ko – E/RT …(vii)

Hence a plot of ln k versus 1/T is a straight line with a
slope of (–E/R). Frequency factor ko is also known as pre-
exponential factor denoted by A.
A catalyst lowers the activation energy of a reaction, but
the magnitude of activation energy for a heterogeneously
catalysed reaction is not automatically accepted to be a
quantitative measure of the catalytic activity. The reason
for not adopting this criterion is the widespread
occurrence of compensation effects [12]. For
heterogeneous catalysis compensation effect may occur
when the overall reaction is a combination of a number of
reactions which show the same mechanism and take place
on different groups of active centres, each group showing
different activation energy and pre-exponential factor [13].
Transesterification reaction consisting of three
consecutive reactions falls under this category. Reaction
velocity constant in this case is a sum of several reaction
steps, and activation energy is termed ‘apparent activation
energy’, Eapp, and pre-exponential factor is termed
‘apparent pre-exponential factor’ , Aapp . In some reaction
systems, it has been observed that the apparent activation
energy varies under different reaction conditions. It is also
found that the variation in apparent activation energy is
accompanied by a change in Aapp, i.e., a large apparent
activation energy is accompanied by a large apparent pre-
exponential factor and vice versa. In some cases, the
changes in apparent pre-exponential factor and apparent
activation energy display a linear dependency according to
the Cremer-Constable relation [14]:
ln Aapp = a Eapp + b …(viii)
The relation implies that values of the natural logarithm of
the prefactor plotted against the apparent activation
energy (denoted a Constable plot), as obtained from an
Arrhenius plot, fall on a straight line with slope a and
intercept b. Although the compensation effect has been
known and studied for a very long time, especially in the
field of heterogeneous catalysis, no consensus on the
nature of compensation effect has been reached.
2. MATERIALS AND METHODS
Transesterification of croton megalocarpus oil was carried
out with methanol using alkaline earth catalysts BaO, SrO,
CaO, MgO, BeO. Heating was provided by a water bath in
studies involving conventional heating, and by a
microwave oven in the studies involving microwave
irradiation. Mechanical stirring was used to ensure
complete mixing, and to eliminate mass transfer
resistances. Experimental setup and procedure has been
described elsewhere [15]. Croton oil was procured from
Help Self-Help Centre, Nairobi. All the reagents were of
analytical grade from Sigma Aldrich. GC standards methyl
heptadecanoate, triolein, methyl myristate, methyl
palmitate, methyl stearate, methyl oleate, methyl linoleate
were from Sigma Aldrich. Equipment included analytical
balance, water bath (Stuart RE 300B, accuracy ± 1oC),
mechanical stirrer (Stuart SS10, 0-2000 rpm), domestic
microwave oven (Shivaki, SMW-103, 1300W), voltage
regulator, magnetic stirrer (Hanna), thermocouple
thermometer (Hanna HI9055), centrifuge (Hettich D-
7200), Teflon
tubing, standard laboratory glassware.
FAME was analyzed using a Gas chromatograph (MRC
GC3420A) with flame ionization detector, capillary column
Agilent CP-Sil 88 (60m x 0.25mm x 0.36mm, coating
0.2μm); carrier gas was nitrogen and other gases were
hydrogen and air. Gases were of analytical grade. Data
analysis was done using Peak-ABC chromatography data
handling system. Heating methods were conventional
heating in a water bath, and microwave irradiation in a
microwave oven. Microwave irradiation is superior to
convective heating as it saves substantial reaction time,
which in turn is a saving of energy [15], [16].
For kinetics studies, a general rate equation for rate of
disappearance of triglyceride, according to overall reaction
(i), was written as,
...(ix)
Here A refers to the triglyceride and B refers to methanol.
The above equation was integrated for m and n ranging
from 0 to 3, for overall reaction order of ‘zero’ to ‘three’.
Integrated forms of the equation [17] were used to
correlate experimental rate data. Highest correlation
coefficient corresponded to the most likely reaction order.
Activation energy and pre-exponential factor was obtained
by plotting ln k versus 1/T according to equation (vii).
3. RESULTS AND DISCUSSIONS
A total of nine catalyst samples consisting of NaOH, BaO,
SrO, CaO, nano CaO, Reoxidized CaO, MgO, nano MgO, and
BeO were used for croton oil transesterification. Sodium
hydroxide was a homogeneous catalyst used for control
purposed. This resulted into eighteen set of data. For the
purpose of reporting the catalysts were coded as A (BaO),
B (SrO), C (CaO), D (nano-CaO), E (Reoxidized CaO), F
(MgO), G (nano MgO), H (BeO) and J (NaOH). Number ‘1’
referred to conventional heating, and ‘2’ used for
microwave irradiation. Table 1 gives the observed reaction
rate constant (k), Pre-exponential factor A, and Activation
energy E for all the nine catalysts, for both conventional
and microwave heating.

Table 1: Reaction kinetics parameters k, A and E.
Run Catalyst k (at 70oC) A E
(kJ mol^-1)
1 A1 1.27E-02 (L^2 mol^-2 min^-1) 3.07E+03 35.38
2 B1 6.19E-03 (L^2 mol^-2 min^-1) 7.37E+01 26.61
3 C1 5.65E-03 (L^2 mol^-2 min^-1) 8.80E+02 33.65
4 D1 1.59E-03 (L^2 mol^-2 min^-1) 2.50E+01 27.35
5 E1 9.80E-04 (L^2 mol^-2 min^-1) 2.53E+01 28.74
6 F1 7.20E-04 (L^2 mol^-2 min^-1) 4.18E+00 24.53
7 G1 5.41E-04 (L^2 mol^-2 min^-1) 1.03E+02 34.26
8 H1 8.70E-05 (L^2 mol^-2 min^-1) 7.17E+00 32.05
9 J1 4.75E-01 (L mol^-1 min^-1) 3.73E+10 71.27
10 A2 5.69E-01 (L mol^-1 min^-1) 2.38E+05 36.77
11 B2 4.88E-01 (L mol^-1 min^-1) 5.67E+06 46.2
12 C2 2.68E-01 (L mol^-1 min^-1) 1.79E+03 36.6
13 D2 3.93E-02 (L mol^-1 min^-1) 4.79E+01 20.49
14 E2 3.35E-02 (L mol^-1 min^-1) 2.44E+07 70.3
15 F2 1.62E-03 (min^-1) 7.46E+00 35.58
16 G2 1.53E-03 (min^-1) 2.99E+02 34.7
17 H2 1.52E-03 (min^-1) 7.86E+1 42.33
18 J2 2.45E+00 (min^-1) 2.68E+06 51
Data in Table 1 was used to plot ln A versus E. Chart 1
gives the resulting Cremer Constable scatter plot.
Solid dots correspond to the data points, and the straight
line is a linear correlation line (R2 = 0.78).
Chart 1: Cremer Constable plot of Eapp and Aapp (Eqn viii)
From Table 1, the chemical reaction constant k was highest
for NaOH catalyst for both conventional and microwave
heating. This means that reaction rates were higher for
homogenous catalyst NaOH as compared to alkali metal
catalysts. For alkali metals, the highest value of k was for
BaO, followed by SrO, Nano CaO, Reoxidized CaO, CaO,
Nano MgO, MgO, and BeO. Hence in terms of activity, the
catalysts were arranged as: NaOH > BaO > SrO > Nano CaO
> Reoxidized CaO > CaO > Nano MgO > MgO > BeO. Similar
observations for alkaline earth oxide catalysts have been
reported by other researchers as well [18], [19]. However,
no similar trend was noted in pre-exponential factor A and
activation energy E. This was suspected to be due to a
large number of simultaneous reactions involved in
transesterification. Observed values of A and E were
therefore ‘apparent’ values for all those reactions. Chart 1
shows that a scatter plot followed a straight line satisfying
Cremer Constable equation (viii). The discrepancy was
attributed to oversimplification of kinetic analysis. All the
reactions were lumped into one expression, equation( ix),
resulting into seemingly irrational data. Reaction velocity
constant was a multiple of A and exp( –E/RT), and it was
observed (Table 1) that where E values were large, A
values were also large, leading to a small value for k. The
activation energies for heterogeneous catalysts were not
found to be directly related to their activities due to the
compensation affect.
4. CONCLUSIONS
Kinetic analysis of transesterification of croton oil with
methanol over conventional and microwave heating using
homogeneous NaOH and alkali earth metal oxide catalysts
gave the highest reaction constant k for NaOH, followed by
SrO, Nano CaO, Reoxidized CaO, CaO, Nano MgO, MgO, and
BeO. This sequence also gave the order of catalyst activity,
highest for NaOH and lowest for BeO. However, no such
trend was noted in values of pre-exponential factor A, and
activation energy E. It was seen that the magnitude of
activation energy was not a direct measure of catalytic
activity for heterogeneous catalysts. This was attributed to
the compensation effect in heterogeneous catalysis. The
overall observed reaction was a result of twenty one or so
reaction steps. This complex reaction mechanism resulted
into irrational values of A and E. The observed values were
thus apparent values, Aapp and Eapp, as given in equation
(viii). Observed compensation effect was due to data
analysis, and not due to a physical phenomena.
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lnA(app)
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Compensation Effect in Transesterification Kinetics of Croton Megalocarpus Oil using Heterogeneous Alkaline Earth Catalysts

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Compensation Effect in Transesterification Kinetics of Croton Megalocarpus Oil using Heterogeneous Alkaline Earth Catalysts